Difference Structures

The difference structure is a powerful data representation technique unique to Prolog. Difference structures simplify and increase the efficiency of programs by permitting ‘partial’ or ‘incomplete’ data structures to be specified and built up incrementally as the program executes. Variables are used as named ‘holes’ that can stand for parts of the data structure that are not yet computed. Difference structures are a generalisation of the idea of an accumulator. Where we have used accumulators to represent the ‘result so far’ during a computation, it is also possible for the idea of the accumulator to be extended to arbitrary data structures.